In the field of plasma (dry) etching, various processing parameters including, for example, RF power, RF bias, pressure, gas flow rate, temperature, and vacuum conditions are pre-determined in order to accomplish a particular plasma etching process. In addition, many plasma etching processes are pre-programmed to follow a series of steps for predetermined time periods also referred to as a process recipe, where the processing parameters may be altered at each step to achieve a desired etching result.
While there are several types of plasma reactor configurations including a wide variety or gaseous reactants used for etching a semiconductor wafer to form semiconductor features, all such processes are generally affected by the concentration of plasma etching species which is in turn dependent on gas supply flow rates and reactor pumping rates. In addition, dry etching processes are dependent on the amount of etchable surface exposed to the etchant (plasma etching species). Since dry etching processes are frequently carried out as steady state processes, reactive species are being created and removed at equal rates when viewed on the scale of the plasma reactor.
The overall partial pressure of the reactive species is inversely proportional to the removal rate of the reactive species which is in general largely controlled by three processes: 1) consumption by etching 2) recombination of the reactive species, and 3) pumping rate of the reactive species or removal rate from the plasma reactor chamber. In addition the partial pressure of the reactive species is directly proportional to the generation rate of the reactive species which, among other factors, is proportional to the gas flow rate. Depending on the particular etching process either the generation rate or the removal rate may have a dominant influence on the etching rate. Several factors affecting the concentration of reactive plasma species may produce etching non-uniformities over the diameter of the process wafer.
For example, one effect, known as the loading effect, occurs where the etch reaction is the dominant removal process. Since the etch reaction is proportional to the amount of etchable surface, the partial pressure of reactive species which is proportional to an etching rate is decreased by an increase in the amount of etchable surface available. Other processes, for example including chlorine atoms may be dominated by the recombination reaction of chlorine atoms to form chlorine gas and may not be affected by an increase in etchable surface area.
Gas flow non-uniformities can affect several mechanisms in dry etching by affecting the local concentration of reactive species over an etch substrate, for example a semiconductor process wafer. For example, in some configurations, more gas arriving at, for example, the process wafer edge may have a higher etch rate compared to the wafer center due to faster replenishment of reactive species at the wafer edge compared to the wafer center. Thus, critical dimension (CD) non-uniformities in semiconductor features vary concentrically from wafer center to edge. Etching non-uniformities including edge to edge non-uniformities also referred to as a leveling effect adversely affect subsequent photolithographic processes by causing defocusing difficulties in transferring mask images to the wafer surface.
A device sizes, including transistors are scaled down to below about 1 micron, CD requirements have become more stringent and difficult to control. For example, two parameters known as bias and tolerance are frequently used to define CD requirements in the semiconductor processing art. CD Bias is the difference in lateral dimension between the etched image and the mask image. CD uniformity is a measure of the statistical distribution, for example 3× sigma, of CD bias values that characterized the uniformity of etching. For example, in etching polysilicon gate structures, the gate length determines the channel length and the acceptable electronic functioning of a transistor making gate CD uniformity critical in the gate formation process. Nonuniform etching rates over the diameter of the process wafer, which are frequently strongly influenced by gas flow characteristics within a plasma reactor chamber, may adversely affect the manufacture of the transistor device in several ways. For example, wafer edge-to-edge (across wafer diameter) variations in etching rates cause leveling non-uniformities which contribute to defocus in subsequent photolithographic processes. As a result, the leveling non-uniformities compound CD bias in subsequent process steps. A goal in the semiconductor manufacture process industry is to achieve CD uniformity to within less than about 30 Angstroms.
A problem in prior art etching processes and systems is the inability to quickly and reliably adjust gas flow control behavior within the plasma reactor system to achieve predictable and repeatable results. Frequently, the gas flow control adjustment process in a ‘black art’ limited by trial and error techniques to obtain the proper gas flow characteristics within a plasma reactor to achieve acceptable CD uniformity. Changes in one of several components of the plasma reactor system over time may unpredictably change etching rates within a plasma reactor chamber. Frequently in order to compensate for altered etching rates, gas flow characteristics must be tuned (adjusted) to compensate for such changes to achieve acceptable CD bias. However, changes in the components that control gas flow characteristics including the reactor pumping speed and/or gas flow supply system may also be affected by time dependent factors such as reactor cleanliness and reactor component aging. Further, etching parameters are frequently required to be altered from one etching process to another, making the re-establishment of optimal etching parameters including optimal gas flow characteristics time consuming and frequently limited to reliance on a trial and error approach.
According to the prior art, efforts to address etch non-uniformities have focused on adjusting the power level of the RF power antenna (excitation source), for example, in an inductively coupled plasma source, a single or dual TCP (transformer coupled plasma), typically disposed outside the reactor chamber adjacent to a dielectric window through which power is transmitted to the reactor gases. In addition, efforts have been made to gain better control of the substrate temperature, for example, by including a dual temperature control on the electrostatic chuck (ESC) that holds the substrate.
It has been found, however, that the gas transport characteristics within a plasma reactor are frequently the most sensitive variable contributing to etch non-uniformities. There have been a variety of gas feed systems proposed for plasma reactors, however many of them are unable to control gas flow characteristics over the diameter of the wafer to avoid or sufficiently compensate for etching non-uniformities. Gas feed systems for plasma reactors have included top gas feed arrangements where the gas feed is fed from the top of the reactor chamber toward the substrate surface including shower head feed arrangements centrally located at the top of the reactor chamber. Further, the prior art has disclosed gas feeds that are centrally located in the upper chamber including one or more gas feeds and which can be directed at a variety of angles generally toward the substrate surface. Gas feed systems according to the prior art, however, lack a system and methodology for selectively controlling gas feed characteristics and/or gas pumping characteristics to selectively control gas flow characteristics in a readily predictable way to compensate for etching non-uniformities. As a result, gas flow control including reactive species concentrations present at the process wafer surface is largely reliant on an uninformed trial and error approach to achieve desired etching uniformity.
Thus, there is a need in the semiconductor processing art to develop a gas flow control system and methodology for reliably adjusting gas flow characteristics within a plasma etcher to achieve repeatable and predictable etching rates including acceptable CD uniformity.
It is therefore an object of the invention to provide a gas flow control system and methodology for reliably adjusting gas flow characteristics within a plasma etcher to achieve repeatable and predictable etching rates including acceptable CD uniformity while overcoming other shortcomings and deficiencies of the prior art.